An incident at a Massachusetts e-cigarette plant landed 29 people in the hospital Sunday, and authorities aren't quite sure why. Getty

An incident at a Massachusetts e-cigarette plant landed 29 people in the hospital Sunday, and authorities aren't quite sure why.

Salem Deputy Fire Chief Alan Dionne reportedly called the scene one of "mass hysteria" as dozens of employees at the Thermal Circuits plant, which produces heaters for items like e-cigarettes, began to complain of breathing problems and nausea.

The factory had been evacuated earlier that same day following a chlorine leak. The shift of around 400 employees had been sent home for the day, according to WCVB.

Authorities say they identified the leak, the source of which was stopped and not restarted by the time the next shift showed up for work.

While the plant was evacuated promptly after the workers began to complain of symptoms, Jennifer Mieth, spokeswoman for the Massachusetts Department of Fire Services, told CBS News that fire crews don't believe the chlorine leak was the cause.

What's more, firefighters said they did not detect chlorine in the building after the second call.

Nonetheless, a little over a dozen people were transported to a Boston hospital and about 15 others drove themselves.

Most were treated and released. Others, including a worker who is believed to have suffered a seizure, remained hospitalized.

As this investigation continues, authorities said the plant would not reopen Monday.